Leetcode: Find Leaves of Binary Tree

Given a binary tree, collect a tree's nodes as if you were doing this: Collect and remove all leaves, repeat until the tree is empty.

Example:
Given binary tree
          1
         / \
        2   3
       / \
      4   5
Returns [4, 5, 3], [2], [1].

Explanation:
1. Removing the leaves [4, 5, 3] would result in this tree:

          1
         /
        2
2. Now removing the leaf [2] would result in this tree:

          1
3. Now removing the leaf [1] would result in the empty tree:

          []
Returns [4, 5, 3], [2], [1].

Better Solution: https://discuss.leetcode.com/topic/49194/10-lines-simple-java-solution-using-recursion-with-explanation/2

For this question we need to take bottom-up approach. The key is to find the height of each node. The height of a node is the number of edges from the node to the deepest leaf.

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public List<List<Integer>> findLeaves(TreeNode root) {
         List<List<Integer>> res = new ArrayList<>();
         helper(root, res);
         return res;
     }

     public int helper(TreeNode cur, List<List<Integer>> res) {
         if (cur == null) return -1;
         int level = 1 + Math.max(helper(cur.left, res), helper(cur.right, res));
         if (res.size() <= level)
             res.add(new ArrayList<Integer>());
         res.get(level).add(cur.val);
         cur.left = cur.right = null;
         return level;
     }
 }

First time solution: HashSet+ DFS

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 public class Solution {
     public List<List<Integer>> findLeaves(TreeNode root) {
         ArrayList<List<Integer>> res = new ArrayList<List<Integer>>();
         if (root == null) return res;
         HashSet<TreeNode> visited = new HashSet<>();
         while (!visited.contains(root)) {
             ArrayList<Integer> leaves = new ArrayList<Integer>();
             helper(root, leaves, visited);
             res.add(new ArrayList<Integer>(leaves));
         }
         return res;
     }

     public void helper(TreeNode cur, ArrayList<Integer> leaves, HashSet<TreeNode> visited) {
         if ((cur.left==null || visited.contains(cur.left)) && (cur.right==null || visited.contains(cur.right))) {
             leaves.add(cur.val);
             visited.add(cur);
             return;
         }
         if (cur.left!=null && !visited.contains(cur.left))
             helper(cur.left, leaves, visited);
         if (cur.right!=null && !visited.contains(cur.right))
             helper(cur.right, leaves, visited);
     }
 }